THE BEST COLD HARDY PLANTS AND TREES FOR YOUR STATEPublication By: Patrick Malcolm

Cold hardiness Importance in Growing plants and Trees.

The lowest cold temperatures during the mid winter onset can vary
somewhat from one year to the next, but over a period of many years
the average temperatures are remarkably stable, however, once every
50 years or 100 years a drastic drop in temperatures may occur. An
example of the sudden, drastic temperature drop occurred in South
Georgia in January of 1983, when it dipped to zero degrees F., and no
one living could ever remember extreme temperatures that low, when
the average low winter temperature here is in the 20's F.

Most climate maps show low and high temperatures that are averaged
over a number of years, however, a random event, like a drastic
temperature drop, like that in South Georgia can happen about every
100 years that caused it to appear that particular winter, when the
thermometer was 0 (zero) degrees F., and that South Georgia was in
zone 6, whereas, it normally is in zone 8 or 9, when the temperature
is normally in the high 20's degrees F.

The effect of a drastic temperature drop to zero degrees, such as
described above that is measured on plants depends on how many hours
that the low temperatures last, before it warms up. Naturally, if
the freeze lasts many hours then the freeze will penetrate deeper
into the soil and more and more root tissue will be damaged or
killed. The time of the day or night is also a consideration when
the low temperature is reach, because of the effect of the sunshine
can warm up the plants.

In order to understand the effect of freezing temperatures on
plants, one must think about the nature of plant cells with each
being surrounded by an enclosed cell wall that is filled with a
complex liquid mixture called “Cytosol”. Within the cytosol
liquid, mostly water, is dissolved a number of different chemical
ions such as Calcium (Ca), Potassium(K), enzymes, protein complexes,
etc. Therefore, one can see that the cell will vary in
intracellular contents from one another in infinite and indefinable
numbers. A plant's 'mass of cells' is made up with numerous cells
that are connected together with many other cells stacked on top of
one another. If one of these cells with its interior liquid is
filled with the complex liquid mixture, ('Cytosol') is compared with
an automobile radiator filled with water – and both the 'mass of
cells' and the radiator are exposed to below freezing temperatures,
it is evident that the fluid (water) will freeze at different
thermometer , depending on what soluble products are contained within
the water. Thus, by adding to the concentration of the antifreeze
to the radiator water, will delay or prevent the freezing and
bursting of the radiator. The same example holds for the plant
cells, meaning that the different concentrations of chemicals within
the plant cells is variable, so that it could lead to the
survivability or the death of the plant like the bursting of the
radiator depending on the concentration of the (Cytosol') Every
plant would show a different tolerance to the freezing temperatures.

The survival of the plant can depend on the environmental factors,
and the rate of plant growth during both the fall growing season and
the spring growing season. During the fall the plant should slow
down in the growth rate. For instance it is well known that the
application of Nitrogen based fertilizers should be reduced or
eliminated, because the nitrogen causes the plant to increase its
rate of growth, thus, the cells are extended and more water is
absorbed, that makes the tender growth much more vulnerable to cold
damage or morbidity. When potassium “K” fertilizers such as KCL
is used, the K ion acts like antifreeze and the plants become
“hardened off” and are much more resistant to the cold damage.
The alkaline effect of the potassium “K” is also effective in
raising the pH of the cells to above “7” which increases cold
resistance. Adjusting a growing plant to the approaching fall
temperatures to force it gradually into a dormant stage can determine
whether or not the plant will survive until the following season, so
that the plant is not stressed and weakened by random temperature
drops that sometimes happen suddenly. When the cells of plants are
swollen or turgid with more than 70% of water, the plant becomes very
susceptible to freezing a bursting.

During the spring the growth of plants can suddenly begin by a
premature warmup, sometimes called an “Indian Summer”, and if a
sudden and drastic temperature drop occurs the plant can be severely
impacted. In the year, 1950, an orchard of Schley and Success pecan
trees that were growing in South Georgia had prematurely leafed out
in February, with some trees showing twig growth of one foot or more,
and on March 20th the temperature dropped to 20 degrees. All the new
growth was killed on the pecan trees took several years to recover to
production. Fruit growers and peach growers are very familiar on how
early spring freezes can kill the flowers that might have produced
peaches.

In the Northern States where snow is common during the winter, the
thick layers of snow flakes actually insulate the plant from extreme
temperatures, mainly due to the air spaces between the snow flakes,
however, ice, can prevent freeze damage for short periods of time,
but eventually after a few hours, freezing can occur.

Grafted fruit trees, shade trees and cloned plants will all behave
similarly when drastic temperatures occur, because all of the genetic
material is identical in each named cultivar. Some exceptions to
this rule do occur, for instance, the rootstock on the grafted plants
all are grown from seedlings (generally) and each root stock has a
different genetic code from the other and thus, a different
resistance to the freezing temperatures/ Rootstocks should be
selected that have a greater resistance to the freezing that the top
grafted scion. Cloned plant cultivars all have identical genetic
cytoplasm and identical freeze resistance.

In the terrible freeze in South Georgia in 1983, when the
temperatures plummeted to zero degrees F., and some large Pindo Palm
trees were frozen, but many that were planted in the same location
that were similarly sized trees showed no leaf damage and survived.
Palm trees, like the Windmill palm, Washingtonia palm and the Sabal
palms that are offered for sale on the commercial market are normally
seed grown and each of these palm trees may show a slightly different
tolerance of cold temperatures, except those that clump and clone
themselves like the Pygmy Date Palm tree (clone) and the Needle palm
tree that will show identical cold hardiness as the mother palm tree.

Sago palm trees also behave in a similar pattern during deep
freeze winters, Most sago palm tree are commercially grown as
tropicals and are grown from the seed, and therefore, react with
individual differences very subtly to extreme frost and some seedling
plants will show no damage and others will be killed by the same
temperatures., Cloned sago palms will react identically to extreme
frosty conditions. In South Georgia, zone 8, Sago palm trees will
often show leaf damage when the temperatures drop into the low 20's,
sometimes partial leaflet damage, sometimes entire damage with brown
leaves that do not regrow until June.

There are other factors that may influence the tolerance of
freezing in plants, such as planting the tree, etc. on the North side
of a building or plant it in some shade.

Nematodes (eel worms) that are found in the South will damage the
roots of a plant and stress will reduce the cold hardiness. Beetles,
worms, and woodpeckers can stress trees, along with even fire ants.